1. Field of the Invention
This invention relates to iron based powder mixtures for powder metallurgy. More particularly, the invention relates to iron based powder mixtures which produce a sintered material excelling in machinability and sliding property, and also provide a sintered material containing Ni, Mo, Cu and the like but having adaptability to sizing even after sintering.
2. Description of the Related Art
In general, powder metallurgy is referred to as a technology in which metallic powder is subjected to pressing in a mold, followed by sintering of the resulting green compact, thereby producing a shape such as a machine part or the like. For instance, iron powder when employed as a metallic powder may be intermixed with Cu powder, graphite powder and the like and then pressed, and sintered to form a sintered material usually having a density on the order of 5.0 to 7.2 g/cm.sup.3. This powder metallurgy technique is capable of producing a machine part of complicated shape with good dimensional accuracy. In order to form a machine part with even more precise dimensional accuracy, however, such sintered material needs in some instances to be further shaped as by machining, drilling or the like.
Sintered material generally has poor machinability and hence causes a short tool life of a machine tool used for machining it, as compared to a wrought steel for example, resulting from rolling of a cast piece produced by continuous casting. Such machining requires added cost.
Poor machinability peculiar to sintered materials is attributable to pores that are present. The pores cause the sintered material to be only discontinuously machined, or reduce the heat conductivity of such sintered material and hence increase the temperature of the portions of the sintered material where it is machined.
Many attempts have heretofore been made to attain improved machinability of the sintered material. These include admixing iron powder with S or MnS. The reason for use of S or MnS is that such material can render machined scrap easily breakable, or can form a thin film of S or MnS on the rake face of a machine tool used. Such a thin film can exert lubricating action during machining of the sintered material.
Japanese Examined Patent Publication No. 3-25481, for example, discloses an iron powder for powder metallurgy which results from atomization of a molten steel with water or gas. The molten steel is composed of pure iron mixed with Mn at a content of 0.1 to 0.5 wt %, and with S, C and the like, and further with S at a content of 0.03 to 0.07 wt %. However, a sintered material produced from this iron powder can improve its machinability by only about twice the corresponding sintered material obtained from an iron powder in common use. Accordingly, a need continues for further improvement.
Moreover, Japanese Unexamined Patent Publications Nos. 7-233401 and 7-233402 teach atomized steel powders each containing S, Cr and Mn. According to the teachings of the two prior publications, a sintered material derived from each such steel powder shows sharp improvement of machinability because graphite remains in the pores of the material, and MnS simultaneously deposits in the iron particles. The reason graphite so remains is presumably because Cr and S prevent graphite from diffusion into the iron particles during sintering of the steel powder.
The resulting steel powder, however, has the defect that sintering in an atmosphere containing H.sub.2 leads to a sintered material having reduced machinability and wear resistance properties. Further improvement has been strongly desired.
Additionally, Japanese Unexamined Patent Publication No. 8-176604 discloses that improved machinability can be gained with an increased amount of graphite by sintering an iron powder comprising boron at a content of 0.001 to 0.03 wt %, Cr in a content of 0.02 to 0.07 wt %, Mn in a content of less than 0.1 wt % and one or more of S, Se and Te in a total content of 0.03 to 0.15 wt %. The technology of this publication allows graphite to remain in the sintered material, but in an amount of approximately 0.42 wt % at the most. A demand has thus been voiced for the development of an iron powder that ensures a larger amount of graphite remaining in the sintered material.
On the other hand, in the production of gears for use as automotive parts by means of powder metallurgy, which gears require high strength and high wear resistance properties, certain alloy elements are commonly added to enhance these properties. For example, Japanese Examined Patent Publication No. 45-9649 discloses adding alloy components such as Ni, Cu and Mo to a pure iron powder through partial diffusion alloying. The steel powder derivable from this production method can produce a green compact having excellent compressibility and sintered steel having strength. However, the resulting sintered material is disadvantageous in that it is excessively hard and is almost completely unadaptable to sizing after sintering, and has poor machinability.